Beautiful work! I suppose these are not the final versions of the units. For the final version, though, I would like to suggest some changes:

-The Marble Gargoyle (upgrade of the Obsidian one) I think it should be white with black/gray marble veins and green eyes. EDIT: This
is a photoshop edit of mine over the M&M 6 Gargoyle. You, of course, keep the shape of the Heroes 3 Gargoyle (with horns and suff), this is just to show what the Marble Gargoyle looks like.
EDIT 2: I also noticed my photoshop image of a Marble Gargoyle looks like stone because of its rough surface(part of the original M&M6 Gif). The Marble Gargoyle should be smooth and shiny (like real marble)

-The final version upgrade of the Iron Golem should be a Steel Golem. Silver->Gold->Diamond Golems should either remain neutral either be an alternative choice for Lv.5 (Instead of Genies.

- The 3rd Mage upgrade looks a little ridiculous with that hat. Why don't you give him a hood so that it looks more similar to the Heroes II Mage (which is also in-universe)

- Naga Queen should be the 3rd upgrade and Naga Warrior(?) as upgrade of the simple Naga.